A contactor may operate according to the principle of an electromagnet. A current in the excitation coil produces a magnetic flux through a ferromagnetic core and a movably mounted ferromagnetic armature. A force acts on the armature at an air gap, as a result of which the armature switches one or more contacts of the switching element. The armature may be reset to the starting position by spring force as soon as the coil is no longer excited. The opening or closing of the switching element is controlled by exciting the coil. As a result of the coil being excited, a movably mounted contact connected to the armature is connected to a fixed contact of the switching element in an electrically conductive manner or is electrically disconnected from the fixed contact. In the closed state of the switching element, the energy supply for the circuit, which is conducted via the switching element, is maintained. In the open state, the energy supply for the circuit, which is conducted via the switching element, is interrupted.
Contactors are being increasingly controlled by a programmable logic controller (PLC). The outputs of this PLC increasingly include semiconductors having a maximum current load of 0.5 or 2 amperes (A) at 24 volts (V) and a DC current. On account of energy and resource savings, contactors are being increasingly equipped with integrated electronic coil controls. These have a very high brief pull-in power, which is constant in comparison with conventional devices. The energy is saved in the holding period. The pull-in power is kept constant by clocking the current through the magnet coil.
Safety-related output assemblies of programmable logic controllers have various diagnosis possibilities, including short-circuit detection or the possibility of diagnosing the interruptions in the lines to the contactor. For the line interruptions, the output assembly reads back the current flowing into the connected device. If the current is below a limit value, the output assembly detects a fault and switches off. However, because the pull-in power of the contactor is clocked by electronics having a very low power consumption, the situation may arise in which the operation of reading back the current falls either in the start-up phase of the connected device or in a pause in the clock cycle. As a result, a line interruption is detected even though the line is okay.
Conventional DC drives without electronic coil control were previously available in the prior art. Current continuously flows through said drives, with the result that fault messages were excluded.
As an alternative to electronically controlled drives, resistors connected in parallel with the device are also used. The disadvantage of this is that the current continuously flows through these resistors, with the result that heating therefore occurs, on the one hand, and unnecessary power loss occurs, on the other hand.